The present invention relates in general to photomask manufacturing, and particularly to locating defects using photomask inspection, review and repair machines.
In conventional photomask manufacturing processes, after a mask is written, developed and cleaned, it is normally inspected on a defect inspection machine and then, if defects are found, reviewed or repaired on a separate machine. Using conventional systems, the information of the photomask inspection such as the defect type and coordinates of the defect can be automatically transferred to the defect repair or review systems. Currently, there are three types of repair techniques: focused ion beam (FIB), far field laser repair which uses optical lenses, and the near field optical repair which uses a micro pipette to deliver the laser beam. Review of photomask defects is normally performed using an optical microscope.
The resolution and accuracy of the x-y translation stage on both inspection and repair machines individually can be as accurate as 5 nm to 10 nm. However, due to the possibility that the origin of the coordinate system encompassing an x-y translation stage is not recorded accurately, calibration between any pair of inspection and repair machines could be off by as much as 40 microns. As a result, using the location coordinates recorded on one machine to find the same defect or other particularized point on another machine may not in itself be enough to locate the point on the other machine. In some cases the defects found on the first machine can not be located by using the first machine""s coordinates on the second machine, even when using the maximum field of view.
According to standard procedures, each repair technique has a distinct method to locate defects found on an inspection machine. For example, the FIB repair machine uses the Ga+ ion beam to scan the mask surface, while the near field optical system uses an AFM to capture the image. The far field laser repair machine will use a lower magnification objective if it is necessary. Several problems arise when defects fall outside the field of view on the repair or review machine. In the case of an FIB repair machine, the Ga+ beam could unnecessarily damage a large area of the mask surface. In addition, the repair/review process is more time consuming on both the FIB and the AFM machines when the operator has to scan for the defects instead of driving directly to the location. Certain types of defects, such as the clear extension or the Cr extension, with size less than 0.2 microns, can not be easily recognized with a low magnification microscope objective such as that used on a far field laser repair machine.
In many cases, the mask will be reviewed on an optical microscope after inspection in order to classify the defect type or to study the detail structure of the defects. Similar to the far field laser repair machine, it is normally very time consuming to find small defects which fall outside of the field of view. What is needed is a more accurate and efficient way to find defects with a mask repair or review device.
The present invention teaches a practical and accurate method for correlating the coordinate systems of independent photomask processing machines. The method of the present invention effectively creates a single coordinate system for the photomask regardless of which and how many processing machines are used. According to one embodiment of the present invention the method for correlating the coordinate systems of two or more photomask processing machines comprises writing a test pattern on a non-productive area of a photomask, mounting the photomask on the first photomask processing machine, automatically searching the test pattern, recording the test pattern in a default coordinate system, reinitializing the coordinate system to a reference point within the test pattern, and storing the location of each of a plurality of points (for example defects in the photomask). The locations of the points are measured relative to the reinitialized coordinate system of the photomask processing machine.
The photomask is then mounted on a second processing machine and its coordinate system is initialized to the same test pattern reference point. The plurality of points are then easily found by applying the stored locations to the initialized coordinate system of the second photomask processing machine. As a result, offset errors resulting from errors in recording the processing machine""s origin, inconsistent photomask placement on the stages, reference placement errors caused by the operator or other sources are not a factor. Processing throughput is thereby improved because the operator no longer has to take additional time to scan the photomask on the second processing machine to locate points which are not at the same location in each coordinate system. The ability to accurately locate points also reduces the damage done to the photomask surface by the Ga+ ion beam when using a focused ion beam photomask repair machine.
According to another embodiment, the test pattern contains at least one reference point at a known location. The photomask is mounted on the processing machine, the test pattern is automatically searched, the location of the reference point is recorded as (Xp, Yp). The locations of particular points (for example defects in the photomask) are identified and stored. The photomask is then mounted on a second machine for the next processing step, the machine is driven to reference point (Xp, Yp) and initialized, and then moved to position (xe2x88x92Xp, xe2x88x92Yp) and reinitialized. The stored point locations are again used to drive directly to the desired points.
In yet another embodiment, the coordinate system on each processing system is initialized by driving the respective photomask processing machine to the reference point at a known location (Xp, Yp) within the test pattern and calibrating the processing machine coordinate system such that the coordinates of the first reference point are (Xp, Yp).
The test pattern and reference point are used for calibrating coordinate systems and may also be used as a reference for other metrologic tasks. In a further embodiment, multiple reference points are used to increase the accuracy of the calibration.